At Pier 63, a Place for the Horsy Set

''Imagine riding in our arena on a summer's eve with the doors up as boats go by,'' said Grania Williams, vice president of operations of the members-only Chelsea Equestrian Center. The 40,000-square-foot indoor/outdoor arena is to open in May at Pier 63, next to Chelsea Piers sports complex, at West 23d Street and the West Side Highway.

''The location is exquisite,'' said Ms. Williams, a member of the Association of Instructors and Fellows of the British Horse Society, as she watched the Hudson splash against the pier.

''I saw a need in the riding community and I'm an entrepreneur,'' said John D'Eri, its developer/owner, who operates World Copy & Imaging Services.

Margaret Work, a principal of its design firm, D-Tek, said the old garage doors would be replaced with mostly glass ones, and the yellow brick facade would remain. Construction is expected to begin in the next few weeks.

A 30-minute supervised session or lesson is to cost $50. Dressage, hunter/ jumper equitation, polo and Western-style instruction will be offered at all levels. There is a $2,500 initiation and $250 monthly fees, with membership limited to 600 and discounts for those who sign up early.

Tishman Real Estate Services Company negotiated the $3 million sublease from Basketball City, with a five-year initial term and two two-year options. (Basketball City, which leases the pier from New York State, is to open a 30,000-square-foot, bubble-enclosed, six-court facility on the roof in April. A marina is to be outside).